as exhibited in its propagation along plates of glass. 6 1 
of a tin vessel enclosing water at a temperature of igo°, they 
polarised a green of the second order. The united thickness of 
these plates was 1.7 of an inch. 
When the heat of my hand was communicated to 11 plates 
of crown glass, they polarised the blue of the first order, and 
exhibited distinctly the two black spaces. The temperature of 
the room during these experiments was 64®. Even one plate 
of crown glass about 0.28 of an inch thick, exhibits the black 
spaces and the bluish white fringes by the heat of the hand. 
The preceding results are neither sufficiently numerous 
nor accurate to enable me to determine the relation between 
the thickness corresponding to the highest tint, and the tem- 
perature of the source of heat. An apparatus, however, is pre- 
paring for me, by which this point will be easily ascertained 
by obtaining various temperatures from heated oil or mercury. 
See Sect. II. 
Proposition XII. 
The number and form of the plates of glass, and the temperature 
of the source of heat remaining the same, the magnitude of the 
fringes of the first exterior set depend upon the law of the decrease 
of temperaiure in that part of the glass which produces them. 
The highest order of colours is always developed where the tem- 
perature is a maximum, and the tints descend in the scale as the 
temperature diminishes. 
Let CDEF, Fig. 5. (PI. II.) be a plate of glass, MN one of 
the black spaces, and the portion CDNM, that which produces 
the first exterior set of fringes. 
The temperatures at the points B, K, G may be represented 
by the ordinates BD, GH, KL, of the curve TLHD. The 
